Lubrication Technology and Increasing the Reliability of Wind Turbine Components

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1 Lubrication Technology and Increasing the Reliability of Wind Turbine Components

2 Roman Vanecek Application Development Specialist Wind Energy Solutions Dow Corning Corporation Abstract In recent decades, the wind energy segment has shown significant growth. The global goal of today is step-by-step increases in the use of modern, carbon-neutral and sustainable sources of energy. To reach this goal, renewable types of energy are needed, and wind energy is expected to become the major contributor. Over the last 30 years, tremendous developments in turbine power and rotor diameter have occurred. In 1983, the typical diameter of a wind turbine was 15 m, with a power output of 55 kw. By 2005, those values had increased to 90 m and 2.5 MW, respectively, for standard turbines. Global cumulative installed capacity reached 200 GW in Wind turbines face harsh environmental conditions that are even more challenging for offshore units. Proper selection of specialty lubricants that allow wind turbines to run with maximum availability by avoiding nonscheduled shutdowns is one of the key contributors to achieving fast return on investment. On the other hand, operating conditions and long maintenance intervals can stress most lubricants to the point that they may interfere with reliability before they can be replaced. With regard to the various rotating parts of wind turbines (e.g., gearbox, bearings, brakes and shrink discs), advancedtechnology lubricants have been demonstrated to contribute to increased component reliability due to their enhanced ability to perform at temperature extremes, resist extreme loads, and protect against corrosion and mechanical wear. An understanding of the key features and benefits of modern lubricants and how to apply them to best realize those benefits can help to extend lubrication intervals, improve equipment reliability and maximize total cost of ownership.

3 Introduction Several different types of rotating equipment compose the typical anatomy of modern wind turbines. There are low-speed parts, such as the main bearings (supporting the main shaft), blade bearings (allowing the pitching of blades), yaw bearing (moving the nacelle against the wind direction) and pitching systems. Generator bearings are typical examples of the highspeed units. The gearbox itself represents slow-speed (10-20 rpm) as well as high-speed (1,500-1,800 rpm) movement. Systems like brakes, shrink discs and various fasteners (e.g., bolts for connection of rotor blades to the hub) typically engage in sliding movement. Requirements for lubricants being used for separation and protection of moving parts are to be specified for each individual type of the application. In general, the requirements include high load-carrying capacity, good protection against corrosion and wear, good run-in lubrication, stability over a broad temperature range, resistance to vibrations and oscillations, compatibility with various elastomers and sealants, and many others. Exact understanding of all requirements may be one of the keys to obtaining the longest, most reliable operation lifetime of a particular mechanical device. Lubricant Selection The best selection and application of a lubricant is ensured by understanding application details, including the component to be lubricated, the type(s) of surfaces in contact (metal-to-metal, metal-to-plastic, etc.) and the component s relative motion (sliding vs. rolling friction). Environmental conditions and one s long-and short-term reliability goals such as extended lubrication intervals, sealing compatibility, cold/hot temperature operation or elimination of corrosion also should affect choice of lubricant. Knowledge of modern lubrication technology enables the prioritization of goals and will allow the best chance to achieve one s objectives. Application details can be summarized by the acronym LETS (load, environment, temperature, speed). Load is an estimation of the amount of force or stress under which the component to be lubricated is operating. Bearings are sometimes referred to as operating under light, moderate or heavy loads. A common rule is that bearings operating at or above 20 percent of their rated load-carrying capacity are considered heavily loaded. Shock or vibration and frequent start-stop cycles are clues that a bearing application is under stressed load conditions. Lubricants chosen for high or stressed load applications usually contain solid lubricants and/or high base oil viscosity. Environment asks the end user to consider the effects of the general climate and surroundings. Among the variables are seasonal and daily temperature fluctuations; atmospheric contamination, including dust, dirt or fly ash; wind; salt spray; and humidity. Substations in warm areas will face different environmental challenges than those in Canada or north China. Offshore environments are more corrosive than onshore areas. Many lubricants used on wind turbine bearings are selected due to their good low-temperature performance; however, in many climates, that benefit is of limited usefulness. Lubricants intended for cold-temperature service often are formulated with low-viscosity base oil to counter the effect of temperature on the viscosity of oil and the consistency of grease. In some cases, what works well in cold environments may not work well in hot environments. Temperature considers the actual temperatures to which the component and lubricant will be exposed, from start-up to normal operating temperature; this is not the same as ambient temperature. An example of temperature would be the difference between ambient air temperature (environmental temperature) at a substation and the actual temperature inside a gearbox at that substation. There may be a significant difference between these two temperatures. Cabinet heaters can generate enough heat to increase the rate of evaporation on greased trip latch and close bearings. Over time, cold or heat from any source could negatively affect greased bearings and may result in slow or no trip or close. Consider grease used to lubricate pitch bearings of the wind turbine. Cold temperatures may stiffen grease enough to delay the grease being pumped into the bearing by the automatic lubricating system. On the other hand, the heat increases the rate of evaporation of the base oil, which can dry and harden grease. Heat damage to lubricants is permanent, whereas grease stiffened by cold temperatures will return to normal when temperature does. Oil viscosity changes with temperature, but oil lost to evaporation or oxidation results in permanent changes to the lubricant. Synthetic lubricants offer improved resistance to evaporation and can operate over wider temperature ranges with longer useful service lives when compared to mineral oil-based lubricants. Speed is a simple measurement of rotational or reciprocating speed how quickly or slowly something moves, as well as how often it moves. Frequency of motion is critical, as static conditions stress grease more than dynamic conditions. Seizure also will become more of a threat under static conditions.

4 Slow-speed and static applications can benefit from lubricants with higher base oil viscosity, as well as those that are fortified with solid lubricants. Using the information in this section, the end user can begin to make some assumptions about his or her application and how it may be best served with lubricants. Applications that move frequently and operate under light to moderate loads outside of severe temperature extremes can be successfully lubricated with liquids the oil in grease or oil itself. As speed or frequency of motion declines, as loads or vibrations increase, and as temperatures fluctuate more into extremes, the end user may wish to explore the possible application benefits of solid lubricants found in some greases, lubricating pastes and dry-film lubricants to solve problems and increase reliability. Component to be Lubricated In a perfect world, it would be easy to lubricate an entire device with one lubricant. In reality, different bearings, gearboxes, slides, threaded connections, o-rings and other components that make up a device may not all be best served with one lubricant. Different types of bearings and gears have been designed to have various strengths for different kinds of service, and they often must be lubricated with different lubricants to help ensure they can deliver their performance benefits. Spindle bearings are lubricated with different lubricants than are tapered roller bearings, and worm gears are lubricated with different lubricants than are spur or helical gears. In certain applications (heavy load, slow speed, higher temperatures), ball or roller bearings may benefit from lubrication with grease that contains lubricating solids like molybdenum disulfide or graphite. Spur and helical gears experience both sliding and rolling friction and can be lubricated with grease and oil with ISO viscosity of 150 to 220 centistokes. Worm gears operate under more severe sliding friction, and the plowing action of movement suggests that a lubricant with higher-viscosity oils is appropriate. O-rings and metal-to-rubber contact applications are wellserved with silicone compounds that can help keep o-rings soft and pliable while providing good lubricity under dynamic conditions. Silicones are compatible with most non-silicone gaskets and o-rings and have been used with success for decades as o-ring and gasket lubricants. These same silicone compounds would not serve well in bearing or gear applications, as they are not fortified to suppress wear in highly loaded metal-to-metal contact applications. Keep LETS in mind as it relates to each component in the device. Certain components are more sensitive to some LETS details than others. Each material of construction may have different compatibility with various lubricants. Many elastomeric materials may shrink or swell to different degrees, depending on the base oil of the lubricant with which they are in contact. Components made with copper, brass or silver may react with or be corroded by lubricants with active sulfur chemistry. Sulfur-based additives often are used in industrial gear oils and some greases. Lubricant Types When most end users think of lubrication, they think of either grease or penetrating oil. Many different lubricants available today fall into one of several categories: 1. Grease 2. Lubricating paste 3. Oil 4. Dispersions of solid lubricants or lubricating oil in solvent 5. Dry-film lubricants Grease typically is composed of 80 to 90 percent oil and 10 to 20 percent thickening agent and also may include 0 to 5 percent additives. Oil provides the lubrication and is a temperaturelimited component. The oil can be one of many fluid types and of any viscosity. The thickener is the reservoir designed to hold base oil in reserve until it is required for lubrication. It is unusual for the thickener to provide lubrication properties itself, but most thickeners also are temperature-sensitive; too much heat can destroy them. It is critical to understand the base oil type and viscosity of a grease, as well as its thickener technology. This knowledge is required to make assumptions regarding the ability of grease to perform in application LETS and allow for good equipment reliability. It also is important in understanding how the lubricant will behave toward certain materials of construction, such as seals and o-rings, and how it may react if mixed with other lubricants. The additive packages of greases are designed to impart or enhance useful properties of finished greases, such as oxidation stability, corrosion protection, low friction or EP load protection.

5 Lubricating paste often looks very similar to grease; both can have smooth, buttery textures and range from white to black in color. Lubricating paste consists of an approximately equal percentage of lubricating solids and low-viscosity oils. Lubricating solids include molybdenum disulfide, graphite, PTFE or powdered metals, among others. Many solid lubricants work synergistically in certain combinations. These lubricating solids often are far less limited by temperature than are liquids. The oil in paste formulations is used for surface wetting and initial adhesion of solids to substrates. It may provide some short-term lubrication but is intended to evaporate quickly and completely under exposure to high temperatures. Paste most often does not utilize thickeners; as a result, solids and oil can separate, especially under shear. Pastes are fortified with additive packages to enhance their ability to provide low friction, good corrosion control, reduction of fretting corrosion, elimination of stick-slip, and long-term service in preventing seizure and allowing for non-destructive disassembly. Depending on application LETS, paste may be used with success on certain bearings, gears, pins and bushings, cams, chains, or knuckles. Heavily loaded, slower-speed sliding friction applications lend themselves well to lubrication with paste, especially if prevention of seizure and long periods of infrequent movement are expected. Paste should serve well in applications where galling, cold welding, fretting or galvanic corrosion may be possible. Lubricating solids used in pastes are excellent at forming continuous lubricating films across contact surfaces; these films are quite effective at separating one metal surface from another, as well as acting as a barrier between water, oxygen and metallic surfaces. These solids are not subject to evaporation or bleed, freezing at low temperatures, gravity, or thermal or oxidative degradation at temperatures up to several hundred degrees. Oil-based lubricants and oil films are subject to destruction by those very means. Solid lubricants offer outstanding lubricating properties at slow speeds, high loads, and sliding and area contact friction. Paste is best utilized by applying a thin layer of product to one or both metal contact surfaces and then either burnishing with a stiff brush or cloth and/or by actuating the device to allow run-in. The act of burnishing or run-in allows for very intimate contact of lubricating solids with the metal surfaces they are used to protect. After burnishing or actuation, application of another very thin layer of paste may provide a good buffer to corrosion and moisture, especially if the device is not expected to move for a long period of time. Keep this layer thin, and remember that it will dry out over time; this is acceptable because the solids provide the functionality. Industrial lubricating oils consist of 95 to 100 percent base oil with 0 to 5 percent additive package. Many properties of finished lubricants are determined by the properties of the base oil and its viscosity. The additive package may be used to lower the pour point; improve the viscosity index; or provide oxidation stability, corrosion protection or some type of wear protection. Many industrial applications utilize oil splash, circulation or bath lubrication. These applications often are in continuous to near-continuous movement at various load conditions and moderate to high speeds ideal conditions to form low-friction fluid films. Failure from corrosion or seizure is possible, but failure from wear or contamination is of higher concern due to continuous operation. Dispersions include, for the purpose of this discussion, aerosols and penetrating oils. The main lubricant is diluted with solvent to aid in dispersing the lubricant over surfaces and to aid its passage into small nooks and crevices. Dispersions usually provide ease of use, but that convenience comes with a price: The solvent carriers that make up the bulk of these formulations are, in many cases, highly volatile and will wash out factory-applied grease lubricants. Fluorosilicone grease lubricants offer resistance to solvents used in aerosols and penetrating oils. Penetrating oils should be used only to free up seized parts and to bust rust. Penetrating oils do not provide any meaningful long-term lubrication, and the bulk of their volumes will remain in service for only a few hours before evaporating completely. Devices treated with penetrating oils will move freely for short time periods, but upon evaporation, the device will return to its sluggish movement or seized condition. The application of penetrating oil can make a situation worse by removing more grease from the device via solvent wash-out. Aerosols are formulated with about 5 to 10 percent actual lubricant, with the remaining content of the can consisting of solvent and propellant. While ease of use is high and the solvents do allow some penetrating action, the end user needs to exercise caution when using aerosol products. It can be easy to apply lubricant beyond the area where it will be effective and possibly mix the aerosol lubricant with another lubricant, which may result in lubricant incompatibility. Dry-film lubricants have found acceptance in automotive component and fastener applications. Dry-film lubricants are applied and formulated similarly to paint: They are brushed, sprayed or screen-printed onto the part in question and then cured by solvent evaporation followed by resin binder cure, which binds a layer of additives to the substrate to which the coating was applied. Once dry, the films can adhere with great effectiveness. One other key benefit of dry-film lubricants is that they dry tack-free; this is a significant benefit, given dust, dirt and other contaminants that can adhere to wet lubricant coatings.

6 Dry-film lubricants work best in applications where speeds are slow and loads are heavy. Many of the most robust dry-film lubricants must be applied in thin, uniform layers and heatcured; the application process is critical to overall success and often is best left to an expert coating shop. The application requirements of dry-film lubricants can limit them somewhat to equipment manufacturers who have their original parts coated before installing them in their equipment. Coating certain pins, gears and threaded parts may offer a chance to reduce failure from seizure and decrease required maintenance for the end user, as these coatings could yield maintenance-free service lives measuring several years to more than a decade. For More Information To learn more about how lubrication technology can help increase the reliability of wind turbine components, visit molykote.com. To contact a Dow Corning industrial lubrication expert, call or wind.energy@dowcorning.com. Conclusion The act of lubrication is no small detail. There are many types of lubricants that offer higher chances of success in equipment being installed in wind turbines. The equipment manufacturer and end user begin to affect reliability of their equipment as soon as they select lubricants with which to service it. These limitations are not evident or often thought of upon initial lubrication or servicing of the device; rather, they present themselves over time as the lubricant is stressed by environmental and static conditions. Better reliability will come from the use of lubricants that do not change (degrade) over time and that are properly applied to the components they are intended to protect from wear and corrosion. Knowledge of various lubricants and their respective strengths and weaknesses will serve design engineers and maintenance professionals well. Better reliability means higher profitability for the end user and speaks to the quality of equipment sold by various equipment manufacturers. Images: AV13141, AV17440 LIMITED WARRANTY INFORMATION PLEASE READ CAREFULLY The information contained herein is offered in good faith and is believed to be accurate. However, because conditions and methods of use of our products are beyond our control, this information should not be used in substitution for customer s tests to ensure that our products are safe, effective and fully satisfactory for the intended end use. Suggestions of use shall not be taken as inducements to infringe any patent. Dow Corning s sole warranty is that our products will meet the sales specifications in effect at the time of shipment. Your exclusive remedy for breach of such warranty is limited to refund of purchase price or replacement of any product shown to be other than as warranted. TO THE FULLEST EXTENT PERMITTED BY APPLICABLE LAW, DOW CORNING SPECIFICALLY DISCLAIMS ANY OTHER EXPRESS OR IMPLIED WARRANTY OF FITNESS FOR A PARTICULAR PURPOSE OR MERCHANTABILITY. DOW CORNING DISCLAIMS LIABILITY FOR ANY INCIDENTAL OR CONSEQUENTIAL DAMAGES. Trademark of The Dow Chemical Company ( Dow ) or an affiliated company of Dow. Dow Corning is a registered trademark of Dow Corning Corporation. The Corning portion of the Dow Corning trademark is a trademark of Corning, Incorporated, used under license. 2011, 2017 Dow Corning Corporation, a wholly owned subsidiary of The Dow Chemical Company. All rights reserved. Printed in the USA SAC6223 Form No: A-01